Nuts and seeds. Try snacking on some seeds and nuts. They contain essential fatty acids that can pop that sheen right back into your hair.
Avocado and banana. Mash a little overripe banana and avocado together, spread in your hair, and leave it there for up to an hour. Then rinse with warm water.
Beer. After your shampoo, rinse your hair with a little beer. This can help restore shine.
Mayonnaise. You'll need the full-fat kind, not a diet or low-fat version. Slather 1 tablespoon or so onto your hair, rub it in down to your scalp, then cover with a plastic cap and wait about 30 minutes. Rinse it out thoroughly or you'll be craving tuna salad the rest of the day.
Oil. Rub a little oil into your scalp. Olive oil works well, as does coconut oil. After you rub it on, cover your hair with a cap and leave it on overnight, then shampoo and rinse the oil out in the morning.